The invention relates to fiber optic coupling.
Fiber optic couplers have been developed for the purpose of splitting the optical power available on a single fiber into several outputs. In a fiber optic coupler, optical power in an input optical fiber becomes distributed among all the optical fibers in the structure. Light not coupled to the neighboring fibers remains in the input fiber. The term "coupling ratio", as used herein, is defined as the ratio of the optical power in any one output fiber to the total output optical power in the coupler.
Fiber optic couplers are fabricated by fusing two or more optical fibers together in a coupling region. Successful coupler fabrication depends upon controlling the placement of the fibers brought into contact with each other in the coupling region prior to fusing. Imperfections such as dirt on the fiber surfaces, imbalances in tension forces applied to the fibers prior to the application of fusion heat, and three-dimensional non-uniformities in the applied fusion heat can reduce the coupler fabrication yield.
Fused couplers have been made from seven cylindrical fibers, in which six fibers surround a seventh central fiber. Due to the circular cross-section of the fibers, six identical fibers can be arranged around an identical central fiber so that each of the surrounding fibers will contact two neighboring surrounding fibers and the central fiber. The term "cross-section", as used herein, is defined as a cut lying in a plane perpendicular to the longitudinal axis of the coupler (i.e., the direction of propagation of light in the coupler). Mutual contact among the fibers promotes uniform fusion, and results in greater output power uniformity. The term "output power uniformity", as used herein, refers to the output power difference between the output fiber with the maximum optical power and the output fiber with the minimum optical power.
As taught by Stowe et al. (1992, U.S. Pat. No. 5,121,452, by one of the inventors here) unitary couplers of the form 1.times.N, with N fibers surrounding a central fiber, may be fabricated if the diameter d of the surrounding fibers is related to the diameter of the central fiber D according to the following diameter-ratio-equation: EQU d/D=sin (.pi./N)/(1-sin (.pi./N)).
For example, couplers employed in distribution systems that require splitting in multiples of four (e.g., 1.times.4, 1.times.8, 1.times.12, and 1.times.16 port configurations) may be fabricated by sizing fibers according to the above equation.